Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A common set-up of a plant for preparing low density polyethylene comprises, beside a polymerization reactor which can be an autoclave or a tubular reactor or a combination of such reactors, further equipment. For pressurizing the reaction components, a set of two compressors, a primary compressor and a secondary compressor, may be used. At the end of the polymerization sequence, a high-pressure polymerization unit may further include apparatuses like extruders and granulators for pelletizing the obtained polymer. Furthermore, such a polymerization unit may also comprise means for feeding monomers and comonomers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.
A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is by far not complete. Per pass of the reactor or the reactor combination, only about 10% to 50% of the dosed monomers are converted in case of a polymerization in a tubular reactor and from 8% to 30% of the dosed monomers are converted in case of a polymerization in an autoclave reactor. The obtained reaction mixture may leave the reactor through a pressure control valve and may then be separated into polymeric and gaseous components with the unreacted monomers being recycled. To avoid unneeded decompression and compression steps, the separation into polymeric and gaseous components may be carried out in two stages. The monomer-polymer mixture leaving the reactor is transferred to a first separating vessel, which may also be designated as high-pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows recycling the ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the conditions of operating the first separation vessel, the polymeric components within the separating vessel are in liquid state. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, which may also be designated as low-pressure product separator, in which a further separation in polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor where these monomers are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream.
The polymerization process in a tubular LDPE reactor is carried out at high pressures which can reach even 350 MPa. Such high pressure requires special technology for the process to be handled in a safe and reliable manner. Technical issues in handling ethylene at high pressures are, for example, described in Chem. Ing. Tech. 67 (1995), pages 862 to 864. It is stated that ethylene decomposes rapidly in an explosive manner under certain temperature and pressure conditions to give soot, methane and hydrogen. This undesired reaction occurs repeatedly in the high-pressure polymerization of ethylene. The drastic increase in pressure and temperature associated therewith represents a considerable potential risk for the operational safety of the production plants.
A possible solution for preventing a drastic increase in pressure and temperature of this type consists in installing rupture discs or emergency pressure-relief valves. WO 02/01308 A2, for example, discloses a specific hydraulically controlled pressure relief valve which allows a particularly fast opening of the pressure relief valve in case of sudden changes in pressure or temperature. It is accordingly technically possible to handle such thermal runaways or explosive decompositions of ethylene within the polymerization reactor, however these situations are highly undesirable since thermal runaways or explosive decompositions of ethylene within the polymerization reactor the lead to a shut-down of the polymerization plant with possible emission of ethylene into the environment and loss of production.
Another threat to the operational safety of high-pressure polymerization plants is the occurrence of leaks. Due to the high pressure difference between the interior of the polymerization reactor and the surrounding, even small fissures in the reactor wall may lead to an exit of a considerably high amount of the reactor content resulting in locally high concentrations of inflammable hydrocarbon in a short time period.
Simply measuring the pressure in the polymerization reactor does not work for identifying leaks in the polymerization reactor because on the one hand the secondary compressor continuously presses new monomer into the reactor and on the other hand the pressure control valve is programmed to keep the pressure in the reactor constant. WO 2008/148758 A1 discloses a method of operating a high-pressure ethylene polymerization unit comprising a tubular reactor equipped with a cooling jacket, in which method the leakage of reaction mixture into the cooling jacket is controlled by monitoring the electrical conductivity of the aqueous cooling medium. Such a method however requires that at least one of the chemical substances in the reaction mixture changes the electrical conductivity of the aqueous cooling medium. Furthermore, leakage can also occur at positions of the polymerization reactor which are not covered by a cooling jacket.
Accordingly, there is a need to provide a process which allows a very fast detection of leaks in a high-pressure polymerization plant and in this way permits to avoid the built-up of explosive hydrocarbon gas/oxygen mixtures. Furthermore, the detection method should be very reliable and trustworthy and should be easy to implement in existing polymerization plants.